NPF:
- Adjust NAT to not assume flow direction in some cases and thus support
less usual setups which are possible when using 'map' with a custom
filter criteria.
- Introduce NPF_SRC/NPF_DST and replace npc_src/npc_dst with npc_ips[2]
for more convenient handling.
- ICMP ALG: restrict matching only to the outgoing traffic, but be more
direction-agnostic elsewhere.